“Preferential” ammonium uptake by sugarcane does not increase the 15N recovery of fertilizer sources

Aims As ammonium (NH 4 + ) has been reported as the “preferred” mineral N source for sugarcane ( Saccharum spp.), the predominant supply of NH 4 + has been hypothesized as a means of increasing the 15 N recovery and crop yield. We aimed to evaluate the 15 N recovery by sugarcane as affected by 15 N-NH 4 + and 15 N-NO 3 − addition. Methods Under field conditions, 15 N-labeled ammonium nitrate [labeling as 15 NH 4 NO 3 ( 15 N-NH 4 + ) and NH 4 15 NO 3 ( 15 N-NO 3 − )] and urea were surface-applied at a rate of 100 kg N ha −1 . Under controlled conditions, sugarcane was grown in hydroponic medium (containing unlabeled N) until the maximum N demand period. Subsequently, plants received 15 N-NH 4 + or 15 N-NO 3 − (as ammonium nitrate). Plants were then sampled at 1, 3, 7, and 15 d after fertilization (DAF) to estimate 15 N recovery. Results Under field conditions, the 15 N recovery for urea was lower than that for ammonium nitrate, partly explained by ammonia volatilization, but there was no difference in N use efficiency between the two fertilizer N sources, as well as between 15 N-NH 4 + and 15 N-NO 3 − supply. Under controlled conditions, however, 15 N-NH 4 + resulted in higher 15 N recovery than 15 N-NO 3 − until 7 DAF. Nevertheless, as 15 N-NH 4 + became depleted in the nutrient solution, 15 N-NO 3 − was taken up more intensely by the crop. At 15 DAF, highest 15 N recovery was detected in the roots and whole plant under 15 N-NO 3 − supply. Conclusions Although uptake of NH 4 + is faster than NO 3 − , the so-called “preference” for NH 4 + by the crop did not necessarily translate into higher 15 N recovery. Therefore, we suggest that the term “preference” should be used with caution to avoid misinterpretation.